One such type apparatus which is available in the prior art is described in German publication DE-PS 25 29 603; this automatic brake adjustment apparatus consists essentially of a worm gear which is in a form-fitting connection with the brake shaft of the vehicle which is engaged with a worm shaft located in a brake lever housing located on the brake shaft.
A drive mechanism having an integrated idle stroke is also located in the housing of the brake lever and is in an active connection with an adjustment mechanism located on the brake lever which is in the form of a lever linkage. The afore-mentioned idle stroke that is integrated in the drive mechanism is in a ratio to a deflection of the brake lever which is proportional to the idle stroke of the brake lining from the starting position until the brake lining comes into contact with the brake drum.
In this prior art brake adjustment apparatus after a specified idle stroke of the drive mechanism, for example. when there is some wear of the brake lining, the worm shaft of the worm drive is rotated by the drive mechanism via a free-wheel device in the form of a one-way coupling. In this manner, the brake lever is thereby adjusted around the axis of the brake shaft. Therefore, the brake shaft is brought into a new starting position relative to an activation mechanism as a function of the wear of the brake lining. The activation mechanism is, for example, a brake cylinder.
The free-wheel device, forming the one-way coupling, comprises a wrap spring in the manner of a coil spring which is located around the longitudinal axis of the worm shaft. Such wrap spring can transmit a turning moment from the drive mechanism to the worm shaft when rotation occurs in the brake adjustment direction.
Such one-way coupling further includes two sleeves located one behind the other on the worm shaft. The two sleeves are surrounded by the wrap spring. A first of these sleeves can be rotated on the worm shaft by the adjustment mechanism as a function of the angular deflection of the brake lever, while the second of these two sleeves can move rotationally in relation to the first sleeve by a specified angle of rotation.
This specified angle of rotation is maintained within its limits by stops which are located between the worm drive and the second sleeve. When the first sleeve rotates in relation to the second sleeve held in place by the worm shaft, in the brake readjustment direction, a friction closing occurs between the outside diameter of the two sleeves and the inside diameter of the wrap spring which is due to the reduction of the diameter of the wrap spring during such rotation. This reduction of the diameter of the wrap spring leads to a compression connection between the two sleeves and the wrap spring. In this manner, a turning moment is established and transmitted from the adjustment apparatus via the blocked coupling which causes a rotation of the worm shaft in the direction of an adjustment of the brakes.
In this prior art adjustment apparatus when the brakes are released, i.e., when there is a relative rotational movement of both sleeves in relation to one another, counter to the adjustment direction, the friction closing disappears so that the one-way coupling can then operate as a free wheel.
The adjustment apparatus allows an adjustment of the brake by hand, for example, for adjustment of the apparatus or when the brake lining is to be changed. For this purpose, the worm shaft can be rotated by an appropriate tool, such as a wrench or a crank. When the worm shaft is rotated in the direction of the loosening of the brake jaws from the brake drum, however, the one-way coupling operates with an increasing friction connection on the wrap spring, in the manner described above in connection with the self-adjustment of the brakes.
Since, however, the turning moment is transmitted in the opposite direction, i.e., from the worm shaft to the adjustment apparatus via the one-way coupling, before the worm shaft is rotated, the force-fit connection between the wrap spring and the two sleeves located on the worm shaft in the manner described above must be overcome. Since the friction closure increases with an increasing turning moment until slipping occurs, undesirable friction fluctuations can occur. These undesirable friction fluctuations may be caused, for example, by variable lubrication conditions or by the manufacturing tolerance of the wrap spring, affecting the tensile force in the winding direction, until the creep limit of the material of the wrap spring is reached. This means that either an undesirable slipping of the worm shaft will be possible, or the wrap spring will rupture.
Of course, the maximum transmissible turning moment can be reduced by a reduction of the friction forces between the wrap spring and the surfaces in a friction closure with it. Such measures, however, would also reduce the minimum transmissible turning moment caused by frictional fluctuations. This could mean that the moment required for the self-adjustment would no longer be available. Ultimately, this would result in a less-precise adjustment of the brakes, or no adjustment at all, caused by a premature slip of the one-way coupling.